This invention pertains to optical systems, in general, and to optical modulators for use in optical systems in particular.
The term xe2x80x9coptical systemxe2x80x9d as used herein refers to any system that utilizes light waves to convey information between one node and one or more other nodes. Much of the optical communications network in place utilizes optical fibers. One property of optical fibers that is of concern is dispersion. Dispersion in optical fiber occurs as a result of variation in the refractive index of the optical fiber with wavelength. Modulation of an optical signal results in optical harmonics of the modulation frequency about the carrier frequency. When modulated light is passed through a length of optical fiber that exhibits chromatic dispersion, the phase of the light at the distal end of the fiber varies as a function of its frequency thus producing phase modulation. Detection of optical signals causes mixing of the various frequency components, but because the various frequency components have differing phases, the mixing results in the amplitude of the detected signal changing on account of the linewidth of the transmitted signal.
In other words, in a dispersive medium, different wavelengths of light travel at slightly different velocities. This causes optical pulses to broaden in wavelength as they travel down optical fiber links, causing difficulty at a receiver when reconstructing an electrical pulse from a received optical pulse. With the advent of erbium doped fiber amplifiers, longer distances are traversed over optical fiber. The problems caused by dispersion are referred to as xe2x80x9cchirpingxe2x80x9d. Chirping becomes increasingly more significant of a problem at higher modulation frequencies such as frequencies at 10 GHz and above. One limiting factor on the length of links in long haul transmission of optical signals is chromatic dispersion that occurs because a transmitter has a real optical linewidth and the refractive index of optical fiber varies, dependent upon the wavelength of the optical signals. Optical linewidth of a transmitter is determined by two factors. The factors are the inherent linewidth at DC and the broadening of the linewidth introduced by modulation. Broadening of the linewidth introduced by modulation is referred to as xe2x80x9cstaticxe2x80x9d chirp. Other optical components may introduce a shift to the center frequency of the optical linewidth; this is referred to as dynamic chirp.
Static and dynamic chirp introduce a pulse width change or phase modulation and an amplitude shift or intensity modulation in the optical signal. The intensity modulation changes are such that there may be a combination of link length, dispersion and frequency that completely nulls out the signal to be detected. In the case of pulse width changes, positive dynamic chirp will broaden the width of a pulse propagating down a fiber and negative dynamic chirp will narrow the pulse. Either of the two effects can render a modulated signal undetectable.
The chirp effects can be compensated for by deliberate introduction of an offsetting chirp in modulated signals. Various modulators providing controlled chirp have been described in the prior art. Typically such modulators are based upon designs that form the modulator on a substrate. The substrate material is frequently lithium niobate (LiNbO3) although other electro-optic materials may be used.
The electro-optic effect in LiNbO3 depends on the direction of the electric field relative to the orientation of the crystalline structure of the substrate along which the optical wave propagates. In an orientation referred to as an xe2x80x9cX-cutxe2x80x9d the optic axis is parallel to the plane of the substrate and at right angles to the direction of propagation of the optical wave, In an orientation referred to as xe2x80x9cZ-cutxe2x80x9d the optic axis is normal to the plane of the substrate.
An optical modulator in accordance with the invention includes an optical waveguide that splits for part of its length into first and second waveguide arms, with the waveguide arms recombining to an output waveguide portion. A first electrode proximate the first waveguide arm subjects a first optical length of said first waveguide to a first modulating electric field. A second electrode proximate the second waveguide arm subjects a first optical length of said second waveguide arm to a second modulating electric field. In accordance with the principles of the invention, the length of the second electrode is shorter that the first length of the first electrode. The lengths of the first and second electrodes are selected to provide a predetermined amount of chirp.
In one embodiment of the invention, the first electrode is one electrode of a first electrode pair proximate said first waveguide arm. The electrodes of the first electrode pair are arranged with respect to each other and the first waveguide arm to subject a first optical length of said first waveguide to a first modulating electric field. The second electrode is one electrode of a second electrode pair proximate the second waveguide arm. The electrodes of the second electrode pair are arranged with respect to each other and to said second waveguide arm to subject a first optical length of said second waveguide arm to a second modulating electric field, the second electrode pair is shorter than the first electrode pair. In accordance with the principles of the invention, the lengths of each of the first electrode pair and the second electrode pair are selected to provide a predetermined amount of chirp.
In accordance with one aspect of the invention, the first electrode pair and the second electrode pair share a common electrode.
In accordance with another aspect of the invention, the gap between the first electrode pair is equal to the gap between the second electrode pair in the regions in which the first and second waveguide arms are subjected to the first and second electric fields, respectively.
In accordance with the principles of the invention, the amount of chirp provided by the modulator is varied from a fixed chirp by varying the power split between the two waveguide arms, and/or the voltage applied to the first and second electrodes and/or a bias voltage applied to bias electrodes.